There has been considerable information published on the production of microbial protein. The term "microbial protein" has developed two meanings. One meaning connotates the whole cell, in which the protein is contained within the confines of the cell wall and therefore is relatively nonfunctional. The other meaning connotates a protein isolated as a separate entity from the microbe. In either case, for human nutrition, the nucleic acid content of the protein product should be reduced to below about 9% by weight, if yeast protein is a substantial source of protein in a human diet. The Recommended Daily Allowance of The Food and Nutrition Board, National Research Council, for protein is 65 grams per day for a 70 kilogram adult male. The Protein Advisory Group of the United Nations System recommends that the amount of nucleic acid ingested per day from microbial protein should be less than 2 grams. Therefore, the nucleic acid content of the protein should be less than 6%, if yeast protein supplies 50% of the dietary protein. The nucleic acid should be below about 3% if yeast is the sole source of protein in the diet.
The nucleic acid content of yeast cells, such as Candida utilis and Saccharomyces cerevisiae, is about 12 to 15 grams of nucleic acid per 100 grams of crude protein. Crude protein is calculated for purposes of this application as the Nitrogen (Kjeldahl) content multiplied by 6.25. The protein isolated from these cells also contain 12 to 15 grams nucleic acid per 100 grams of crude protein. Thus, the nucleic acid content should be reduced several fold before a substantial amount of the protein is used for human nutrition. The nucleic acid of yeast is mainly ribonucleic acid or RNA, and in this application these terms will be used interchangeably.
In U.S. Pat. No. 3,887,431, a process is described whereby yeast protein concentrate containing a low level of ribonucleic acid (RNA) is produced by degradation of the yeast RNA by the ribonuclease of the yeast itself. This patent teaches that it is more desirable to extract the yeast protein from broken yeast cells under highly alkaline conditions than under lower pH conditions such as neutrality.
It is well known that various metal ions are required for the activation or stabilization of various enzymes. However, it has not been known until this discovery that the presence of zinc salts or biologically available zinc at certain critical concentrations stabilizes yeast ribonuclease against the effect of alkaline conditions, that is, a pH of greater than about 7. The zinc is required to assure that the nucleic acid accompanying the protein is degraded by the yeast ribonuclease to nucleosides sides and nucleotides. Zinc salts or biologically available zinc may be added to the yeast growth broth or at any of several operational points in the process used to prepare a yeast protein concentrate having a low nucleic acid content.
Yeast growth processes are known (U.S. Pat. No. 3,616,249) which use high levels of zinc compounds to optimize yeast yield when the yeasts are grown on hydrocarbons. However, the yeast growth processes of the present invention do not require the addition of high levels of zinc compounds to achieve satisfactory yeast yields.
When food yeasts are grown on molasses, beer wort, or cheese whey, there is sufficient zinc native to the substrate itself to provide for an optimal yield of yeast. Such yeasts do not have sufficient zinc content to protect the ribonuclease of the yeast against degradation by alkali when the yeast cells are ruptured and the protein extracted under highly alkaline conditions.